Many wireless devices such as mobile telephones require oscillation circuits to generate high frequency signals that may be mixed with incoming received signals and mixed with outgoing signals to be transmitted. In the case of mobile telephony, a given mobile telephone might have to operate across a broad range of frequencies or channels in order to avoid interfering with other mobile telephones.
To provide this broad range of frequencies, a frequency synthesizer is often employed. A frequency synthesizer typically includes a controlled oscillator circuit, such as a voltage controlled oscillator (VCO) that receives a control voltage or signal and, in response thereto, alters or shifts the frequency of the VCO signal output to a desired frequency.
In some implementations an oscillator will employ a tuned circuit comprising an inductor (L) and capacitor (C) to provide an LC tank circuit that is resonant at a particular frequency. In order to change or shift the frequency of the signal, the inductance and/or the capacitance can be changed. One way to do this, for example, is to connect, in parallel with the capacitor of the LC tank circuit, a voltage-variable capacitor (i.e., a varactor) whose capacitance changes in the presence of an applied voltage. Thus, when the control voltage changes, the capacitance of the overall LC tank circuit changes, and, as a result, so does the resonant frequency of the LC circuit, thus causing a change in output frequency.
While such VCO implementations can be used in many applications, such circuits cannot typically be used in applications such as mobile telephony, in which very precise and discrete signal frequencies are needed to, among other purposes, avoid the potential for interference mentioned above. Accordingly, digitally controlled discrete addition of capacitance has become increasingly popular in modern frequency synthesizers.